Thermogravimetric analysis (TGA) is a widely established technique for measuring the change in weight of a sample as a function of temperature or time in a controlled atmosphere. The change in weight is usually accompanied by the evolution of gases caused by sample decomposition or evaporation. TGA is used to study the thermal stability of materials, decomposition kinetics, and moisture and volatiles content as well. However, TGA does not have the ability to identify gaseous components evolved during the weight loss. The identification of the gaseous components in these off-gases would be beneficial in determining sample characteristics such as decomposition pathways or thermal stability.
Infrared Spectrometry (IR) is used extensively in areas of gas analysis because it is a relatively convenient technique and provides positive identification of gases. For example, IR spectrometry is used in applications such as gas chromatographic (GC) infrared analysis to identify vapor phase materials. In GC/IR spectrometry, a gaseous sample is passed through a flow cell analyzer, also known as a light pipe. The various constituents of the sample flow from the GC at different times. Infrared radiation energy from an interferometer is directed through the light pipe and is absorbed by the constituents in a manner which determines the quantity and type of constituent. The infrared radiation from the sample is then directed into a detector. In the course of operation, the interferometer includes a movable mirror which reciprocates back and forth and produces interference within the infrared radiation so that during each scan or movement of the mirror, the output of the detector produces an interferogram. This output is an electrical signal in analog form that is then amplified and digitized in the electronic system and fed into a data system where it undergoes a Fourier transform and is analyzed. A description of Fourier transform infrared spectrometer (FTIR) systems may be found in Griffiths, Peter R., "Fourier Transform Infrared Spectrometer", Science, Vol. 222, pp. 297-302, 21 October 1983, the contents of which are incorporated herein for an explanation of the FTIR system.
A typical light pipe accessory used in FTIR includes a hollow glass tube with infrared transparent windows sealed at either end of the tube. Infrared transmissive "salt" windows, e.g., windows made of potassium bromide or zinc selenide, are mounted to both ends of the light pipe using seals to seal off the bore of the light pipe to prevent escape of the gases. The characteristics of the light pipe are crucial to the performance of the system. It is generally desirable to maximize the number of sample molecules that are in the infrared beam path while minimizing the radiation loss due to reflection and absorbence. Material in the light pipe which contacts the gas must also be non-reactive. The light pipes used in commercial instrumentation to meet these requirements are typically cylindrical glass tubes having a small internal diameter on the order of 1 to 3 mm and which have a thin coating of gold deposited on the inner surface. Gold is used because it is reflective, stable and inert.
In the past, the TGA and FTIR analysis systems were used independently and often by separate operators in different laboratories. This, however, has proved to be an inefficient means for providing a complete sample analysis, or for measuring quantitative weight loss and identifying evolved gases.